Heretofore, various processes for producing so-called fruit extracts have been developed and employed, wherein fruits are heated for extraction in the presence of added water to give an extract, which is then concentrated into a juice.
For example, prune extract obtained by extracting plums, particularly dry plums as raw material, and concentrating the resulting extract, has recently been widely marketed and consumed.
Most of the prior art processes for producing prune extract and the like have been generally carried out by first adding water to fruits as the raw material, and cooking this material at atmospheric pressure, and subsequently concentrating the resulting extract; or as a variation of the above process, by continuously feeding steam into fruits dipped in water so as to cook such fruits to thereby obtain a concentrated extract.
However, with such various conventional processes, the time required for the extracting treatment often exceeds several hours and is often of the order of 9 hours. Thus, they have a disadvantage of a very high energy cost. Further, they have the drawback of inefficiency in that with such a long processing time, greater attention from operating personnel is required. Moreover, they have the additional drawback in that treatment without any cover, i.e. so-called open type treatment, usually employed for operational efficiency, causes the hazard of the unhygienic mixing of extraneous matter into the resulting product.
In the case of fruit extracts, it is desired that the extracts be made into a pasty juice through the modification of pectin contained in the fruits as raw material. Also, for increasing the yield and further shortening the extraction time and thereby reducing the energy cost, means for removing seeds, etc. in advance is inherently desired. This however inevitably injures the fruit. As a result, undesirably, the extraction of pectin in view of the injury to the fruit is too rapid and the resulting extract jellifies and forms an extract having a considerably inferior quality to that of the above-mentioned desired product.
The reason for the above undesirable results is that in case of the injured, so-called diced or flaked fruits, their sarcocarp contacts directly with the extraction liquid and the pectin substance readily jellifies in the form of sugar and acid.
Thus, not only the use of the above-mentioned diced or flaked fruits as raw material, but also uninjured fruits, or socalled "whole" fruit, fails to help the situation, and hence there is an inherent drawback in the prior art of a high cost with respect to all types of raw material. Moreover, there is a problem raised with unripe whole fruit. In case of "unripe whole" fruit, unlike the case of "ripe whole" fruit, pectin is yet in the form of protopectin, which combines with calcium and minerals to form a salt, which is integrated with cellulose to maintain a hardness in the form of cells. Thus, there is the danger that in the above-mentioned treatment in hot water, the pectin modification will be insufficient to make the viscosity higher in the form of sugars and acids whereby a jelly is formed. Accordingly, the use of overripe whole fruit wherein pectin has been modified into pectinic acid which is water-insoluble and stable, is desirable. However, in dealing with raw material, it is nearly impossible to detect the degree of ripeness of whole fruit, or to screen overripe whole fruit from unripe whole fruit, and even if this may be possible, there occurs a disadvantage in that a high-level screening apparatus is required, resulting in a high cost.
In addition, although ripe fruits contain mainly pectin, they have a disadvantage in that they jellify as well by virtue of the water-soluble property of pectin.